Rubio is not expected to attend the group's first meeting on Jan. 27, but he went further in a recent interview, saying that the caucus could co-opt the grassroots conservative movement.

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"My concern is that politicians all of a sudden start co-opting the mantle of Tea Party. If all of a sudden being in the Tea Party is not something that is happening in Main Street, but rather something that’s happening in Washington D.C.," he said in an interview with the Shark Tank, a Florida political website. "The Tea Party all of a sudden becomes some sort of movement run by politicians. It’s gonna lose its effectiveness and I’m concerned about that."

But membership in the group might not become a litmus test for Republican lawmakers. Some conservative bloggers have echoed Rubio's concern about the caucus.

"When Representative Michele Bachmann spearheaded the Tea Party Caucus in the House, I had mixed emotions. When one person or a group of people claim to represent ideas, then the whole movement can be tainted by the missteps of the individual," Melissa Clouthier wrote at the conservative blog RedState in a post about Rubio's membership.

Rubio was an early underdog in the Florida Republican Senate primary behind Gov. Charlie Crist. But he eventually picked up momentum with the help of the Tea Party and drove Crist out of the race, and later the GOP. Rubio defeated Crist, who ran as an Independent, and Rep. Kendrick Meek (D) in the general election.

The Florida senator and conservative favorite said he would be joining the Senate Steering Committee, an existing group of conservative senators.

"Why do we need something in addition to the steering committee?" Rubio asked. "And there maybe a good reason. I'm curious to hear about that."